1. Field of the Invention
The present invention is related to a DC transformer, and more particularly to a DC transformer with an output inductance integrated on a magnetic core thereof and a DC/DC converter employing the same.
2. Description of the Prior Art
A typical transformer has a magnetic core and two insulated windings. When an alternating current flows to a primary winding, magnetic flux lines perpendicular to the windings in the magnetic core provide a voltage in a secondary winding, the voltage being directly proportional to the number of windings of the secondary winding. Therefore, with an appropriate winding number, a desired voltage can be obtained.
Referring to FIG. 11, this is a schematic diagram of a prior art DC/DC converter 11. The DC/DC converter 11 includes a DC transformer 10. The DC transformer 10 includes a primary winding and a secondary winding wound around a magnetic core 12. The primary winding receives an input voltage Vin through an input filter 14 and a half-bridge converter without PWM (Pulse Width Modulation) control function 41, while the secondary winding outputs a voltage Vo provided by the magnetic core 12 of the DC transformer 10 through a synchronous rectifier 42 and an output filter 16. If a turn ratio between the primary winding and the secondary winding is N, and the duty cycle of the DC transformer 10 is D, the output voltage Vo can be expressed by Vo=(D/2N)Vin.
FIG. 12 is a schematic diagram of the magnetic core 12 and the primary and secondary windings of the DC transformer 10. The DC transformer 10 includes a primary winding 13, a first secondary winding 15, and a second secondary winding 17. The magnetic core 12 is an EE type core, which includes three legs (Leg1, Leg2, Leg3). The winding operation for the primary winding may be performed by winding the primary winding either around a first leg (Leg1), or around a second leg (Leg2) and a third leg (Leg3). The primary winding 13 shown in FIG. 12 is wound around Leg1. Correspondingly, the secondary winding can be wound in different manners. The secondary winding shown in FIG. 12 is wound around Leg1, with a portion of the secondary winding being extracted out from either side of a center tap 16. The first secondary winding 15 and the second secondary winding 17 are thereby formed.
FIG. 13 is a circuit diagram of the DC/DC converter 11 of FIG. 11. The DC/DC converter 11 includes the input filter 14 (C1, L1), the half-bridge converter without PWM control function 41 (capacitors C2, C3, switches Q1, Q2), the DC transformer 10, the synchronous rectifier 42 (switches Q3, Q4), and the output filter 16 (inductor L2, capacitance C4). The DC transformer 10 includes the magnetic core 12, the primary winding 13, the first secondary winding 15, the second secondary winding 17, and an auxiliary winding 19. The input filter 14 is used to filter the input voltage Vin. The primary winding 13 is connected to the half-bridge converter without PWM control function 41 (formed by the switches Q1, Q2 and the capacitors C2, C3), wherein the half-bridge converter 41 is used for driving the DC transformer 10. The first secondary winding 15 and the second secondary winding 17 are respectively connected to the switches Q3 and Q4 of the synchronous rectifier 42. The auxiliary winding 19 provides an operation voltage for the switches Q3 and Q4. The output filter 16 comprises the output inductor L2 and the output capacitance C4, and is used for filtering the voltage provided by the DC transformer 10, resulting in an output voltage Vo.
Usually, the above-described DC transformer includes an output filter provided by an output inductance and an output capacitor, for suppressing noise persisting in the output voltage. However, the output inductance may occupy a large area on a printed circuit board. The output inductance also has a tendency to dissipate power, which lowers the efficiency of power transformation.